Safety circuit for compressor drive



3,102,677 FOR COMPRESSOR DRIVE S-AFETY CIRCUIT Robert J. Evans, Kankakee, and Gerald G. De Mocko, Bradley, 11]., assignors to A. 0. Smith Corporation,

Milwaukee, Wis, a corporation of New York Filed Sept. 17, 1959, SerrNo. 840,760 Claims. (Cl. 230-6) This invention relates to a safety circuit for a refrigerant compressor drive and is particularly directed to a safety circuit including means responsive to the pres sure or temperature of the refrigerant to discontinue operation of the compressor incident to predetermined abnormal conditions.

vIn refrigerating units, air conditioning units and the like, a compressor is employed to compress the refrigerating fluid or medium and establish circulation of the fluid in the circuit. The compressor thus has a high pressure discharge and a suction or'low pressure inlet.

A high-low pressure safety switch is normally coupled to both the high pressure side-and the low pressure side of the compressor. The temperature of the refrigerant isproportional to the pressure of the refrigerant and consequently the switch can be either pressure or temperature sensitive. Although the. subsequent discussion refers to the pressure sensitive switch which is the more widely employed type, the invention is also adapted for temperature sensitive switches in water chiller and similar devices.

If the high pressure increases above or the low pressure locked in a latch circuit. with a manual reset lever such that the restarting of the compressor requires some posirtive action by the operator in order to restart the compressor.

The restarting of the compressoris desirable by an external switch such that the consumer can readily operate the unit without calling a service man. Repeated stopping of the compressor indicates the necessity of a service call. Generally, certain prior devices employed the main contactor relay connected in series circuit with the reset relay. The relay design was consequently interdependent. Further, certain prior art structure employed a continuously energized reset relay which resulted in periodic failures with the resulting increase in replacement and maintenance cost as well as unnecessary inoperative p e-- riods of the compressor.

The structure of the present invention provides independent contactor relay and reset relay circuits with the latter only energized during predetermined abnormal operation of the compressor.

The present invention provides a simple circuit employing a dual pressure switch which is connected respectively to the highand low pressure sides of the compressor. The pressure switches are connected in a control circuit in series with a thermostat unit to actuate an electroresponsive compressor contactor. 'Ilie compressor is thus controlled in response to the state of the thermostat.

The dual pressure switches are also connected in a reset and latch circuit with an electroresponsi-ve safety device and the thermostat. The switches normally hold the latter circuit open. If either pressure switch is actuated, the safety device positively breaks the circuit to the compressor motor and holds the circuit broken until 3,102,677 Patented Sept. 3, 1963 subsequent actuation of the thermostat, or reset switch,

breaks the latch circuit. The pressure switches are thus connected in circuit with both the compressor and the safety device such that if either or both of the pressure switches are actuated the circuit to'the compressor motor is broken and the relay reset device is simultaneously operated.

The thermostat or reset switch is actuated to open the circuit to the compressor and consequently to the safety device and thereby returns the complete circuit to standby. The thermostat or switch is again turned to the cooling position which closes thenormal circuit to the compressor. However, if the abnormal condition remains, the pressure switches are soon actuated to energize the safety device and the compressor again stops. A continued requirement for restarting of the compressor indicates that further service is required to correct the malfunction.

The dual pressure switch in accordance with the present invtntion can be individually connected in circuit to the safety device such that either or both of the switches are eit'ective. Thus,- where it is desirable to have only one or the other of the switches in circuit, a simple electrical connection or disconnection can be employed to cifect the desired result.

The drawing furnished herewith illustrates the best mode presently contemplated for carrying out the invention. I

In the drawing:

FIGURE 1 is a schematic circuit diagram of an air conditioning control unit incorporating a dual pressure safety switch of the single pole, double throw variety;

FIG. 2' is a view similar to FIGURE 1 employing a double pole, double throw switch for the dual pressure switch; and

FIG. 3 is a view similar to FIGURES 1 and 2 employing a single pole, single throw safety switch coupled vto the air conditioning unit in accordance with the present URE 1, a refrigerating device such as an air conditioning invention.

Referring to the drawings and particularly to FIG- unit 1 is diagrammatically shown comprising a compressor 2 coupled to and driven by a motor 3. The refrigerating cycle itself is of any conventional design and is therefore not shown. A thermostat d is connected in series with a safety control circuit 5 to operate and control a magnetic contactor or starter 6 which is connected in circuit with the motor 3 and a set of power lines 7.

The motor 3 is any conventional variety or type which is adapted to drive the compressor and establish circulation of the retrigerant, not shown. .The connection of motor 3 to power lines 7 is controlled by thestarter or contactor 6 which in turn is responsive to the cooling demand recorded by the thermostat 4.- p

The illustrated starter or contactor 6 is shown as a conventional relay having a pair of sets of contacts 8 which are serially connected in circuit one set with each of the power lines 7 and the motor 3. An armature 9 is biased to an open contact position to open the sets of contacts 8 and break the circuit to the motor 3. A winding 10 is magnetically coupled to the armature 9 and is connected in circuit with the thermostat 4 and the safety circuit 5. Whenever there is a demand for cooling, the thermostat 4 completes the circuit to the winding '10 which is energized and moves the armature 9 to close the sets .of contacts 8 and to thereby complete the circuit to the motor 3. The motor 3 then continues to operate until such time as the sets of contacts 8 open. I

The thermostat 4 is diagrammatically illustrated as incorporating an on-off switch 11 which is adapted to be manually positioned to turn the air conditioner 1 on and oif. A bi-metal adjustable switch 12 is connected in series in compressor 2.

. type switches.

1 the air conditioning unit 1 which is over-ridden by'the safety circuit incident to abnormal pressure conditions of the motor 3 incident closing of bi-metal switch 12.

However, if the dual pressure switch 13 is actuated by either the high or low pressure actuators 21 and 22 the circuit to the relay winding 6 iscompleted and the circuit to the contactor 26 is .open to prevent enengiz-ation of the motor3.

The connection of the dual pressure switch 13 to accomplish the previously described operation of the circult is as follows:

The safety circuit 5 includes a dual pressure switch 13 and a reset relay 14 which are connected in a control I circuit with the thermostat 4 and the contactor 6.- A stepdowntransformer 15'connect's the control circuit 5 to the powerlines 7 in the illustrated embodiment of the invention to establish a suitable low voltage control circuit. Although a line voltage control circuit can be used, the

low voltage control circuit permits the use of small, in-

expensive and long life components and substantially eliminates any danger of shock incident to malfunctioning or breakdown of the insulation of the components.

Generally, the dual pressure switch 13; and the reset relay 14 are connected in circuit allowing normal opera- 7 tion, of the starter tor contactor 6 in response to the status of the thermostat 4. The dual pressure switch 13 is responsive to the pressurecf the refrigerant on either side of the compressor to actuate the reset relay 14 and open the circuit to the motor 3. The switch 13 is responsive The dual pressure 'switch 13 includes a high'pressure switch 16 and a low pressure switch \17 which are both A jumper lead 27 connects the normal condition contact 19' of the high pressure switch 16 to the switch arm 18 of the low pressure switch 17. The switch arm 18 of the high pressure switch 16 is connected in series to the thermostat switch 12. The normal condition contact 19 of the low pressure switch 17 is connected in series with the normally closed reset relay contacts 24 and the winding 16} of the contactor 6. Consequently, whenever the on-off switch 11 is closed and the bi-metal switch 12 closes, the contactor winding 1t)v is energized as follows:

The current flows from the transformer 15 through the thermostat 4 to the dual pressure switch 13. The current flows from contact arm 18 of high pressure switch 1'6 and the engaged normal condition contact 19 through the jumper lead 27 and then through the compressor switch 17 to the normally closed contacts 24, through I r normally closed contacts .24 of relay 14 and then through the-contactor winding 10 back to the transformer 15.

The winding 10 is consequently energized and the armato abnormal pressure conditions to actuate relay 14 and prevent subsequent actuation of the compressor motor 3 3 until the thermostat 4 or switch 11 has beenopened.

illustrated in FIGURE 1 as single pole, double throw 7 Y The switches 16. and 17 each include a switch arm 18 which is biased into engagement with a normal condition contact 19. Each switch arm 18 is adapted to be moved trom the normal condition contact 19 into engagement with an abnormal condition contact 20. The switch arms 18 are respectively connected to a highpressure actuator 21 and a low pressure actuator 22 which are connected respectively to the high pressure and the low press'uresides of the compressor 2.

' The pressure actuators 21 and 22 are conventional and well-known devices and are therefore shown diagrammatically; Thus, the actuatorsmay taker-the form of a capillary, tube, not shown, which is adapted to actuate suitable bellowsor diaphragms, not shown, and thereby move the switch arms .18 in response to predetermined pressure variations. i

I fir the absence of any abnormal pressure variations the pressure actuators 2 1 and 22 leave the dual pressure switch 13' in the normal state during which motor 3 is under the control of the thermostat 4.

,In the presence of any abnormal pressure variations,

the dual pressure switch :13 is actuatedas subsequently described .to energize the reset relay 14 which locks open the circuit to the motor contactor 6 and prevents energization of the motor 3.

The reset relay 14 is, shown diagrammatically as a v conventional relay having an armature 23 adapted to simultaneously control a set of normally closed contacts 24 and a set of normally opened contacts 25 A relay winding 26 is associated with the armature 23 and connected'in circuit with the dual pressure switch 13 to selectively position armature 23 to open and close contacts24'and 25.

The contacts 24 are connected in series with the contactor winding 10 to establish a complete circuit to the contactor winding 10 whenever the thermostat 4 is moved to close the circuit. I Normally, the winding 26 is de-energized and the contacts 24 are closed to allow euergization ture 9 moved to close the contacts 8. (lonscquently the motor 3 is energized by power taken from the power 111168 .7 to drive the compressor 2 and establish the desired The compressor 2 under'normal condicooling elfect. I tions operates until thermostat bi-metal switch 12 opens. A lead 28 connects the abnormal condition contacts 20 of the high and low pressure switches 16 and 17 to each other and to one side of the reset relay winding 26 of reset relay 14. A lead 29 connects the opposite side of the relay winding 26 directly to the transformer-15.

' If an abnormal pressure variation is established, either switch arm 18 of the high pressure switch 16.01 of the low pressure switch 17 is actuated by the corresponding actuator 21 and 22 to move the corresponding arm into j I engagement with the associated abnormal conditions con-- Y tact 20. The relay winding 26 is energized and the con- 1 tactor winding 10 is simultaneously de-energized as follows: The movement of either arm 18 fromthe associated normal condition contact 19 breaks the previously described circuit to the 'contactor winding 10.

The circuit to the relay winding 26 is then completed through the dual pressure switch 13 and thelead 28;

The energization of relay winding 26 moves the associated armature 23 which opens'contacts 24 and closes contacts 25. I breaks the circuit to the relay winding 10* of contactor 6 and prevents energization of the compressor motor 3 until the relay winding 26 is de-energized and the. armature 23 returned to the normal position.

The relay contacts 25 create a latching circuit to the winding 26 incident to initial actuation of energization of the winding. A lead 30 connects one of the contacts 25 to the thermostat side of the dual pressure switch-13 and a lead 31 connects the opposite of the contacts 25 a to lead 28' and thus to the opposite side of the dual pressure switch 13. The contacts 25 by-pass the dual pres-* sure switch 13 and maintain independent energization of the relay winding 26.

Current flows from the'transformer 15 through the a closed thermostat 4 and then through the now closed contacts 25, by passing the dual pressure switch 13. The current flows-from the .contacts 25 directly through the lead 28 and winding 26 and back Itothe transformer 15.

The relay winding 26 is therefore energized until such time as either the on-off switch 11 or the'bi-metal switch 12 is opened to break the circuit to the relay winding 26 and return the armature 23 to the normal position.

The opening of the contacts 24 positively transformer 15 common to the opposite side of relay winding 26.

The operation and the embodiment of the invention illustrated in FIGURE 1 issummarized as follows:

Assume the on-off switch 11 has been closed to turn the air conditioning unit 1 on. Further, assume that the temperature in the adjacent area has risen to a level which establishes a demand for cooling and therefore the bi-metal switch 12 has been moved to the closed position shown in FIGURE 1.

Current then flows from the transformer 15 through the thermostatd and the dual pressure switch 13 which is in the full line position shown in FIGURE 1. The current flows through the normally closed contacts 24 of the reset relay 14 and then through the contactor winding back to the transformer 15. The winding 10 electromagnetically drives the armature 9 to close the contacts 8 and complete the power connection of the motor 3 to the power lines '7. The motor 3 then'drives the I compressor 2 to establish the desired cooling effect.

. Consequently, armature 9 returns to the normal open position and immediately de-energizes the motor 3.

requires service. Suitable steps can then be taken to obtain professional service.

The immediately above-described summation of the operation of FIGURE 1 assumed actuation of the high pressure switch 16 by actuator 21. Actuation of the low pressure switch 17 by the low pressure actuator 2?. es ta-blishes a similar functioning of the apparatus.

When the low pressure switch 17 is actuated the arm 18 disengages the associated normal contact 19 and thus breaks the circuit to the contactor relay winding 11) to immediately establish-de-energization of the motor 3. The movement or the anrn 18 of switch 17 into engage ment with the abnormal condition of contact 20' directs the current again to the lead 28. When the low pressure switch 17 is actuated, the current [from the thermostat 4 first flows through the normally closed or engaged arm 13 and contact 19 of switch 16 and then via the jumper lead 27 to the arm 18 to the abnormal condition contact 20 and to lead 28 to energize the reset winding 26. The current flow is otherwise as set forth in the description for actuation of switch 16.

The illustrated switches 16 and 17 can have the switch 18 and associated contacts 19 reversely connected to the thermostat 4 and to the reset relay 14.

If it is desired to cut out the operation of the reset relay by. either the high or the low pressure switches 16 or 17, the lead 28 is removed from the desired terminal 20. The circuit then responds to-pressure variations only The movement of arm 18 to contact 20 directs the current from the transformer 15 and thermostat 4 to the lead 28.

' The current fiows to the lead 28 and the connected relay winding 26 directly back to the transformer 15' via lead 29. As previously described the energization of 'winding 26 opens contact 24 to positively break the circuit to the contactor winding 10 and also closes the normally open contact 25 which is connected in a latch circuit to maintain energization of the winding 26.

Lamp 32 is illuminated whenever relay 26 is energized to establish an immediate visual indication that a mal- I functioning of the compressor has occurred and that the compressor motor 3 is de-energized.

The windingZd and lam-p 32 are maintained energized until either the on-ofi switch 11 or the thermostatic switch 12 opens to break the latch circuit and return the relay 14 to the normal position.

Normally, the switch 12, which was closed in response to a demand for cooling, does not reopen Ibecause the compressor has been turned oil and the required cooling has not been provided.

To-restart the compressor. and again operate the air conditioning unit 1, the manual on-off switch 11 is turned to the 011 position and then turned backto the on of the compressor. I

1f the abnormal high pressure condition, however, returns, the circuit is broken as previously described. Frequent requirements for restarting of the compressor motor indicates that the equipment is malfunctioning and in the direction of the switch contact ltlwhich remains connected to lead 28.

Referring particularly to FIG. 2 an embodiment of the present invention which employs a dual pressure switch having a double I throw, double pole construction is shown. Corresponding elements in the embodiments shown in FIGURE 1 and FIG. 2 are given corresponding ntnnbers for purposes of clarity and simplicity of description.

Each of the switches 16 and 17 of dual pressure switch 13 in the embodiment of FIG. 2, includes a set of nor mally closed contacts 33 and a set of normally opened contacts 341. A switch arm 35 normally bridges the normally closed contacts 33 and is adapted tobe moved into bridging engagement of the normally open contacts 34-. Each of the jumper arms 35 of switches 16 and 17 is connected to the corresponding high and low pressure actuators Z1 and 22 for selective positioning in accordance with the pressure conditions on opposite sides or" the compressor 2. v a

The dual pressure switch 13 of FIG. 2 is connected in circuit in the following manner:

A umper lead 36 connects corresponding contacts of the normally closed contacts '33 of switches 16 and 17.

A lead 37 connects the opposite normally closed contact of SWltCh 16 to one side of the contactor winding A lead 38 connects the opposite normally closed contact 33 of switch 17 to one of the normally closed contacts 2 2 of the reset relay 14. A lead 39 connects the opposite side of the normally closed contacts 24- to a jumper lead 40 which is connected to the thermostat 4.

With the switch arms 35 in the normal position as shown in full line in FIG; 2, a circuit is established to the contactor winding 1% to operate motor 3 incident to closing of thermostat switches 11 and 12 as follows:

Current flows from the transformer 15 to the thermostat 4 and'then to the dual pressure switch 13. The current flows through jumper lead 411 and lead 39 to the normally closed contact 24. From the normally closed contact 24, the current flows through the normally closed contacts 33 01 the switch 17 and the switch 16 via lead 36 and then through the relay contactor winding 10' back to the transformer 15.

The lead 41] connects corresponding contacts of the normally open contacts 34. Ajurnper lead 41 connects the opposite contacts of the normally open contacts 34 to one side of the relay winding 26 of the reset relay 14.

.46 in effect shoots the pressure switches 16 the invention. I I A lead 48 connects one of the relay contacts '25 to the lead 47 and a lead 49 connects the opposite contact Otf Consequently, if either switcharm 35 of the switches 16 and 17 is actuated by the corresponding actuator 21 and 22 to disengage contacts 33 and to engage the associated contacts 34, the previously described circuit to the contactor winding isaopen.

. A circuit to the relay winding 26 is completed as follows: The current from the thermostat 4 is impressed directly upon the lead 41 either through the contacts 34 of switch 16 or through the contacts 34 of the switch 17 due to the. jumper connection lead 40. The cunrent flows through contact lead 41 to and through winding 26 and then directly 'back to the transformer 15.

A lead 42 connects one of the latch contacts 25 to the lead 41 and a lead 43 connects the opposite contact to the lead 39. Consequently, after contacts 25 are closed, current flows directly (from the thermostat' t to winding 1 26 through the leads 39 and 43 and then through contacts switch for each'of the switches 16 and ll'l of the dual pressure switch 13. URES '1, 2, and 3 are-given corresponding numbers (for purposes of clarity and simplicity of illustration.

Referring particularly to FIG. 3, each of the switches 16 and 17 includes a switch arm 44 which is adapted to selectively engage and disengage a contact'45. A lead 46 connects the arms 44 to each other and [directly to the thermostat 4 and to one sideof the normally closed contacts 24 of the reset relay 14. The contactor relay winding 10 of contactor 6 is thereforeenergized as long as the contacts 24 are maintained closed because the lead the circuit. I

A lead 47 connects contact 45 of switches 16 and 17 together and to one side of the reset winding 26. The opposite side of winding 26 is connected to the trans- [former 15 as in the previously described embodiments of the relay contact 25 tothe thermostat connecting lead 46. The latch contacts 25 are thus connected to maintain energization of the relay winding 26 subsequent to initial energization. I I

The operation of the embodiment of the invention shown in FIG. 3 is generally similar to that of the previous embodiments. In the embodiments of FIG. 3, the

contactor winding 1% is normally maintained energized through the circuit comprising the transformer 15, the thermostat 4, the jumper lead 46, the normally closed .contacts 24 and the relay Winding 10.

However, if either of the switch arms 44 is actuated by the associated pressunefactuator 21 and 22 moves into engagement with the corresponding contact 45, the circuit to the relay winding 26 is completed. The circuit includes the thermostat 4, switch arm- 44 and associated engaged contact '45 directly or through lead 46, lead 47 to winding '26 and rthenback to trans-former 15. When the relay winding 26 is energized, the normally closed contacts 24 open to break the circuit to the contactor 6 and the latch relay contacts 25 closed to maintain energization or [the relay winding 26. The relay winding 26 is maintained energized until such time as either the onoffswitch 11 or the bi-metal switch 12 of thermostat 4 is opened to break the latch circuit and reset the apparartus'to standby condition.

The illustrated circuits all describe a single phase power connectionifor purposes of simplicity and clarity of illus- Corresponding elements-in FIG- s tration. However, the invention is equally applicable to a polyp'hase or other electrical power system.

The control circuit may also be employed to 'llhus, gasoline engine driven units and the like may: be controlled by the circuit of the pnesent invention.

The present invention by the use of a dual pressure switch allows selective connection of the high pressure safety switch and the low pressure safety switch into the circuit. V

The series circuit connection allows simple remote control through the conventional thermostat without th necessity for any additional switching. Various modes of carrying out theinvention are contemplated as being within the scope iof the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention. H

We claim:

1. In a safetycontrol circuit for operatively connecting and disconneoting'a compressor to 'a power source,

an ion-ofif switch to selectively condition the compressor for operation, a temperature responsive switch means in series with the on-oii switch to restrict compressor operation to preselected temperature conditions, elect-roresponsive reset means having contact means connected in series with said on-oif switch and said temperature responsive switch means, a dual pressure switch means having. a first set of contacts. adapted to be coupled to the high pressure side of the compressor and a second set of contacts adapted to be coupled to the low pressure side of the compressor and having said first and second set of contacts connected in circuit with the reset means for simultaneous and individual control of'the reset means,

and latch circuit means connectedtosaid electroresponsive resetmeans'to maintain enengization independently of I the: dual pressureswitch asa result of the initial enerigiz'ation oi the electroresponsive reset means by operation and 17 from of said dual pressure switch means.

.2. In a safety control circuit for an electrically con trolled compressor, an on-ofi? switch'to selectively=oondition the compressor for operation and establish an input pressure level at one side and an output pressure level at the other side thereof, a temperature responsive switch means in a series connection w'ith the on-off switch to restrict compressor operation to preselected temperature conditions, a first double pole and double-throw switch having a first set and a second set of contacts alternately closed by a switch arm adapted to be coupled to one side of the compressor and responsive to the pressure level to selectively dispose said arm in engagement with the associated sets of contacts, a second double-pole and double' throw switch having a first set and a second set of con-' inen'ga-gement with the associated sets of contacts, an' electroresponsive reset'means having a set or reset contacts, means connecting said first sets or contacts in series with said switches and said reset contacts to conjointly control said compressor, and means connecting said second sets of contacts in a parallel-series circuit with said switches and said electroresponsive reset means to actuate the reset means.

3. In a safety control circuit for an electrically con- I trolled compressor, an on-off switch to selectively condition the compressor [for operation and establish an input pressure level at one side and an output pressure'levelat the other side thereof, a temperature responsive switch means in a series connection with the on-off switch to.

of thecompressor and responsive to the pressure level to selectively dispose said arm in engagement with the assocontrol other'lthanqelectric motor driven compressor circuits.

. of contacts in series with said switches and saidreset contacts to conjointly control said compressor, means connecting said second sets or" contacts in a parallel-series circuit with said switches and said eleotroresponsive reset means to actuate the reset means, and means connecting said latching contacts directly between said temperature responsive switch means and said reset means to shunt said second and first double-pole and double-throw switch. 1

4.*In a safety control circuit rfor an electrically controlled compressor, an on-off switch to selectively condition the compressor :for operation and establish an input pressure level at one side and 'an output pressure level at the other side thereof, a temperature responsive switch means in a series connection with the on-01f switch to restrict compressor operation to preselected temperature conditions, an electroresponsive reset means having a set of normally closed contacts connected in series with the temperature responsive switch means and the on-oft switch to control the compressor and having a control winding, a first pressure responsive switch having a switch armadapted to be coupled to one side of the compressor and to engage a contact incident to a preselected pressure variation of the input pressure level, a second pressure responsive switch having a switch arm adapted to be coupled to the opposite side of the compressor and to engage a contact incident to a preselected pressure variation of the output pressure level, and circuit means connecting said switches in parallel with each other and in seriesparallel with said control Winding and said temperature responsive switch means and the on-oti switch means.

5. In a safety control circuit for an electrically controlled compressor, an on-otf switch to selectively condition the compressor for operation and establish an input pressure level at one side and an output pressure level at the other side (thereof, a temperature responsive switch means in a series connection with the on-off switch to restrict compressor operation to preselected temperature conditions, an electroresponsive reset means having a set of normally closed contacts connected in series with the temperature responsive switch means and the on-off switch to control the compressor and having a control winding, a first pressure responsive switch having a switch arm adapted to be coupled to one side of the compressor and to'engage a contact incident to a preselected pressure variation, a second pressure responsive switch \having a switch arm adapted to: be coupled to "the opposite side of the compressor and to engage a contact incident to a preselected pressure variation of the input pressure level,

said switches being connected in parallel with each other and in series-parallel with said control winding and said temperature responsive switch means and the on-oif switch means, and a set of normallyopen contacts connected in parallel with said switches and actuated by said electroresponsive means to latch the latter in circuit.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN A SAFETY CONTROL CIRCUIT FOR OPERATIVELY CONNECTING AND DISCONNECTING A COMPRESSOR TO A POWER SOURCE, AN ON-OFF SWITCH TO SELECTIVELY CONDITION THE COMPRESSOR FOR OPERATION, A TEMPERATURE RESPONSIVE SWITCH MEANS IN SERIES WITH THE ON-OFF SWITCH TO RESTRICT COMPRESSOR OPERATION TO PRESELECTED TEMPERATURE CONDITIONS, ELECTRORESPONSIVE RESET MEANS HAVING CONTACT MEANS CONNECTED IN SERIES WITH SAID ON-OFF SWITCH AND SAID TEMPERATURE RESPONSIVE SWITCH MEANS, A DUAL PRESSURE SWITCH MEANS HAVING A FIRST SET OF CONTACTS ADAPTED TO BE COUPLED TO THE HIGH PRESSURE SIDE OF THE COMPRESSOR AND A SECOND SET OF CONTACTS ADAPTED TO BE COUPLED TO THE LOW PRESSURE SIDE OF THE COMPRESSOR AND HAVING SAID FIRST AND SECOND SET OF CONTACTS CONNECTED IN CIRCUIT WITH THE RESET MEANS FOR SIMULTANEOUS AND INDIVIDUAL CONTROL OF THE RESET MEANS, AND LATCH CIRCUIT MEANS CONNECTED TO SAID ELECTRORESPONSIVE RESET MEANS TO MAINTAIN ENERGIZATION INDEPENDENTLY OF THE DUAL PRESSURE SWITCH AS A RESULT OF THE INITIAL ENERGIZATION OF THE ELECTRORESPONSIVE RESET MEANS BY OPERATION OF SAID DUAL PRESSURE SWITCH MEANS. 